So lately I've been experiencing this really strange, almost faint grinding sound when at low speeds at I take my foot of the gas. It sounds like you're going really really fast in reverse that high pitch rubbing sound. I can barely hear it in the cabin, however when I roll the window down it's completely obvious. Anyone have any ideas?

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2006 BMW X5 4.8is Lemans Blue/Beige/Dark Maple
-Nav, Comfort Seats, Heated Steering Wheel,
Rear Airbags, AUX Input
2009 BMW M3 Space Grey/Black/Carbon
-all options